Field
The disclosed technology relates generally to magnetic devices, and more particularly to spin torque majority gate devices such as spin torque magnetic devices (STMG), and to methods of fabricating the same.
Description of the Related Technology
As traditional logic circuits based on CMOS continue to scale in physical dimensions, the power consumption per unit area can also correspondingly increase. As an alternative to logic circuitry based on CMOS, what is known as spintronics has been proposed. In spintronics, magnetization of devices is used to generate the computing signals, instead of electric charge-based signals used in CMOS logic circuitry. Using properties of spintronics, a type of device referred to as majority gate devices can be built, e.g., spin torque majority gate (STMG) devices. A STMG device is a logic device whose output depends on the majority of states of its multiple inputs.
In an article of Nikonov et al. entitled “Proposal of a Spin Torque Majority Gate Logic” as published in IEEE Electron Device Letters 32 (8) in August 2011, a possible layout is disclosed of a spin majority gate device with perpendicular magnetization. As described herein, a perpendicular magnetization refers to a net magnetization of a ferromagnetic layer whose direction is aligned along a direction (e.g., z direction) that is perpendicular to a plane (e.g., x-y plane) formed by ferromagnetic layer. An example layout of a SMTG device is schematically illustrated in FIG. 1 (a top-down plan view), whose cross-sectional side view is illustrated in FIG. 2. The illustrated spin torque majority gate device 1 has of a common free (switchable) ferromagnetic (FM) layer 202 (on a substrate 100) and thereon four independent fixed FM layers 2181, 2182, 2183, 2191 in three nanopillars 218 and one nanopillar 219, respectively. The three nanopillars 218 are configured as inputs, and the one nanopillar 219 is configured as an output. The free FM layer 202 and fixed FM layers 2181, 2182, 2183, 2191 are separated by a non-magnetic spacer layer 204. The arrows designate the magnetization directions. The operation of the device is based on spin torque transfer. The device operates by applying a positive or negative voltage to each input nanopillar 218, which thereby determines the directions of current flowing through each nanopillar 218 and the resulting spin torques. The corresponding logic state for each input nanopillar is the logical “0” and “1” for a negative of positive input voltage respectively. The free FM layer 202 can thus also be in one of the two states of magnetization, up (not shown) or down (shown by the arrow 2021, being ‘down’ in FIG. 2), depending on the input signal and corresponding to the logical “0” and “1” output signal. Spin torques act to retain the gate in its state or to switch it to the opposite state, depending on the free FM layer magnetization direction and the voltage applied to the input pillars. The majority of the torques determines the outcome (i.e. output signal). The state of magnetization is detected by measuring the tunnel magnetoresistance (TMR) between the free 202 and the fixed layers 2191 by the output pillar 219.
Top-pinned magnetic tunneling junctions (MTJs) with perpendicular anisotropy (PMA) can be advantageous for the input and output pillars due to their improved scalability for smaller feature sizes.
However, fabricating such a magnetic stack poses challenges. Some top-pinned MTJs need to be partially etched to enable a freestanding common free layer 202 having the PMA that is shared between the different input 218 and output 219 pillars. For processing such top-pinned MTJs, the etch process should stop selectively on the non-magnetic spacer layer 204, which can be formed of MgO with a thickness of about 1 nm. In such processes, any residual magnetic or conductive material on top of the non-magnetic spacer layer 204 can be detrimental. However, etch species can penetrate as much as a few nanometers, and can thus easily penetrate into or through the non-magnetic spacer layer 204. Damage to the non-magnetic spacer layer 204 and to the interface between the common free layer 202 and the non-magnetic spacer layer 204 can cause a loss of PMA, a loss of tunnel magneto resistance (TMR) and result in non-functional STMG devices.
There is thus a need to overcome these issues and provide improved STMG devices.